Vehicles incorporating multiple drive axles benefit in many ways over vehicles having a single driven axle. Drive axle systems in such vehicles may be configured to distribute torque proportionately or disproportionately between the axles. Additionally, shift mechanisms may be provided to such vehicles to permit the disengagement of one of the driven axles, and to transition from single axle operation to multiple axle operation during normal vehicle operation, among other benefits. However, such versatility typically requires the incorporation of additional drive train components into the vehicle at added expense and weight. Such added weight results in a decreased fuel efficiency of the vehicle.
Clutching devices in such drive axle systems also need to be selected based on a gear reduction ratio present in a wheel differential. Axle ratios may be of a two-speed configuration to permit the vehicle to operate in a low speed and high torque manner or in a high speed and low torque manner. It is preferred to drive multiple axles when the low speed and high torque manner of operation is desired (to distribute the higher torque amongst a greater number of wheels) and it is advantageous to operate a single axle when the high speed and low torque manner of operation is desired (to decrease windage and frictional losses when torque distribution is of lower concern). However, incorporation of both the two-speed configuration, an axle disconnect function, and an inter-axle differential may be prohibitive with respect to cost and weight. Such added weight, windage losses, and frictional losses result in a decreased fuel efficiency of the vehicle.
When multiple axles of a drive axle system having the inter-axle differential are operated in the low speed and high torque manner of operation, torque output at each of the axles should optimally be equal to prevent slippage of the axle having a greater torque. The inter-axle differential having a planetary style differential, by design, unequally divides torque. As a result, the inter-axle differential having the planetary style differential, when used with multiple drive axles having similar axle ratios, can slip as a result of unequal torque distribution when the vehicle having the inter-axle differential is operated on a low friction surface.
It would be advantageous to develop a drive axle system that is lightweight, reduces windage and frictional losses, can be operated in a low speed and high torque manner of operation and a high speed and low torque manner of operation without excessively increasing a cost of the drive axle system.